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Re: filling delay slots with branches

From: Jeff Law <law at redhat dot com>
To: apl at alum dot mit dot edu
Cc: Alan Lehotsky <alehotsky at me dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
Date: Wed, 06 Mar 2013 06:47:12 -0700
Subject: Re: filling delay slots with branches
References: <42B2405E-2519-488D-94F7-759EF228D40C@me.com>

On 03/05/2013 08:00 PM, Alan Lehotsky wrote:

Am I correct in my understanding that you can't put a branch instruction in the delay slot of a branch instruction?

Semantically, the HW I'm looking at annuls the branch in the delay slot if the first branch is taken, but any other instructions are not annulled; but it appears that there's no way to describe this in the define_delay() and it looks to me like the delay-slot for the instruction in the delay slot won't get filled properly either.
e.g.
         cmpi $r1,0
	jeq  $1
            jlt  $2
              jmp  $3
                nop
would be a 3-way branch  on zero, neg or  (by elimination) positive values with  the indented instructions being
in a branch delay slot.

I'm not aware of any way to describe it to the current delay slot scheduler; I looked at it briefly eons ago as the PA allows branches in delay slots, but gave up -- for the PA at least the semantics weren't very useful.

jeff

References:
- filling delay slots with branches
  - From: Alan Lehotsky

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